Hydrodynamic forces – moving water cause hydrodynamic forces which cause frontal impact, water striking front of a house. Drag effects, as water runs along side a house. Eddies or negative pressure, as water the down stream side of a house.
Debris impact – floods will and can move logs, vehicles, lumber, propane tanks, which often hit homes. Items that don’t float are dragged and rolled in the floodwaters.
Hydrostatic forces – is the weight of standing that puts pressure on a structure. Most houses were not built to withstand lateral pressure. Most walls will collapse in 3 feet of standing water. That is why homes required to get flood vents cannot have the bottom of the opening no higher than 12 inches at grade. Building that are not anchored correctly will actually float away due to hydrostatic pressure.
Soaking – in standing water plywood, gypsum wallboard will fall apart. Wet wood will swell and crack if dried to fast.
Contaminants & sediment – Most materials used to build house such as wood, fiberglass, and cellulose, will absorb floodwater and sediment. Flood waters can carry sewage waters, animal carcass, and bacteria. After a house is dried and repaired it still may be unhealthy because the materials have absorbed sediment and bacteria.